Porous materials, namely mesoporous, microporous, and macroporous materials, are interesting classes of materials useful for various applications. [1] The discovery, in 1992, of periodic mesoporous silica materials denoted M41S or MCM-type having pore sizes 2-10 nm represented a paradigm shift in the synthesis of porous materials.
The materials were prepared in a straightforward synthesis that involved the aqueous phase co-assembly and acid or base-catalyzed hydrolytic poly-condensation of silicate-surfactant micelles followed by removal of the surfactant by thermal or chemical or photochemical post-treatment steps. This synthetic strategy created a silica replica of the templating micelles (a silicatropic mesophase) and represented a new way of creating silica-materials with crystalline mesoporosity, having a pore architecture (i.e., hexagonal, cubic, worm-hole) and pore dimensions (2-10 nm) that were predicted upon the structure and dimensions of the surfactant-directing micelle assembly. Using this synthetic approach the mesopore dimensions could be chemically controlled with angstrom precision. In an extension of this strategy researchers demonstrated that by using tri-block copolymer micelles, involving for example the co-assembly of a polypropylene oxide-β-polyethylene oxide-β-polypropylene oxide mesophase with silicate precursors, as a new and larger dimension templating mesophase, then the mesopore size range of the MCM41 class of periodic mesoporous silica materials, could be boosted to the upper mesoscale range of 10-30 nm to create a class of much larger mesopore silica materials, denoted as SBA periodic mesoporous silica. [2]
It is important to note that the channel walls of all these MCM and SBA classes and structure types of periodic mesoporous silicas were glassy having just short range order, the channel walls lacked structurally well-defined silica sites like those found in zeolites (a class of solids defined as crystalline microporous aluminosilicates), and were found to be devoid of useful channel functionality for perceived applications that could benefit from the size and shape-controlled mesopores and specific adsorption properties of the materials. In other words, while the mesopores in MCM41, MCM48 and SBA materials were monodispersed (single size) and the mesoporosity could be either periodic (hexagonal, cubic) or randomly organized (worm-hole), [3] the material behaved more or less like any other form of porous silica sol-gel type chemistry, exemplified by the well-known classes of materials called xerogels and aerogels, and that contained a random spatial distribution of different diameter mesopores in a glassy silica matrix. Hence the envisioned benefits of this new class of periodic mesoporous silica materials were never really realized in practice, and, to the best of our knowledge, no products or processes have emerged in more than 10 years since their discovery. Tremendous efforts have been devoted to overcome the functionality deficiency of the MCM41, MCM48 and SBA class of mesoporous silica materials by, incorporating other elements into the materials, creating entirely different compositions, crystallizing the constituents of the channel walls, by incorporating useful organic functionality into the materials. [4]
In the context of functionalization two main methods of integrating organic function into periodic mesoporous silica to create hybrid organic-functionalized mesoporous materials have been devised:
A) The first involving the grafting of organo-functionalized alkoxysilanes RSi(OR)3 to the external or internal surface silanol groups SiOH to give the desired organic-functionalized mesoporous materials; with “external” meaning the external wall of the particles and “internal” meaning the channel walls with the functionalization inside the channels. [5] The functional groups can therefore be covalently anchored after the formation of the mesoporous material or incorporated into the template to react inside the channels during the synthesis of the material. Whichever synthetic strategy is used to make these organo-functionalized mesoporous materials with organic groups terminally bound to the walls of the channels, the surfactant template can be removed from the material by thermal or chemical or photochemical post-treatment steps.B) Another way to functionalize the framework of the porous material involves the use of a silsesquioxane-type silica precursor (exemplified by (OR)3SiRSi(OR)3) in which the organic function R instead of being present as a terminally bonded group to the alkoxysilane is rather positioned as a “bridging group” between two alkoxysilyl groups. [6] The resulting templated material is called a periodic mesoporous organosilica (PMO) in which the bridging organic group R is exclusively integrated into the silica framework to create organosilica channel walls.
The ability to directly include, in a predetermined fashion, bridging organic groups into the silica walls of a periodic mesoporous silica was pioneered by Inagaki. [7] His work opened the basis for an entirely new class of PMO nanocomposites, synthesized from the “bottom-up” and with “molecular scale” control, and which offered a myriad of envisioned opportunities based upon the ability to utilize organic synthetic chemistry to control the chemical and physical properties of the material.
The ease of functionalization and the versatile morphology manipulation of the PMOs allowed the development of several hybrids, which can likely be exploited to advantage in a number of application areas including but not limited to controlled release of chemicals and drugs, chemical sensing, bioassays, catalysis and separations, to name a few. [8]
There is, however, still some doubt regarding the question of the degradability of this materials. In fact, once exploited the material for a predetermined purpose, degradation/breakdown of the material does not spontaneously occur, sometimes causing the instauration of accumulation issues (i.e. biomedical applications) or the performance of costly, and not always efficient purification procedures to remove the particles from their working environment.
It would be very advantageous to provide a method of producing an entirely new class of hybrid porous organometaloxide (HPO) materials that have all the desired attributes of the PMOs but are able to overcome the degradation issue mentioned above. Thus, one objective of the present invention is to provide a new class of hybrid porous organometaloxide (HPO) materials characterized by a self-destructive behavior, in order to have better control of their fragmentation and dissolution, avoiding, or at least decreasing the risk of accumulation and facilitating elimination.